1. Field of the Invention
The invention relates to a process for preparing polyvinyl acetals, where the mother liquor from a preceding batch is used in the form of the aqueous medium in which the acetalization takes place.
2. Description of the Related Art
The preparation of polyvinyl acetals, which are obtained from the corresponding polyvinyl alcohols by polymer-analogous reaction with the appropriate aldehydes, was disclosed as early as 1924, and since then a wide variety of aldehydes have been used for preparing the corresponding polyvinyl acetals. Polyvinyl acetals are prepared in a 3-stage process (polyvinyl acetate→polyvinyl alcohol→polyvinyl acetal), and the resultant products contain vinyl alcohol units and vinyl acetate units as well as vinyl acetal groups. Polyvinyl formal, polyvinyl acetacetal, and polyvinyl butyral (PVB) have achieved particular commercial importance. The term “modified polyvinyl acetals” hereinafter means polyvinyl acetals which contain other monomer units as well as the three previously mentioned units vinyl acetate, vinyl alcohol, and vinyl acetal.
The largest application sector for polyvinyl acetals is the production of safety glass in automotive construction and for architectural uses, plasticized polyvinyl butyral films being used as an intermediate layer in glazing units. Another application sector for polyvinyl butyrals is use in corrosion-protection coatings, found in EP-A 1055686, for example. Polyvinyl butyrals are also used as binders in coatings, and specifically in printing inks, one of the reasons here being their good pigment binding power. Examples of these are the modified polyvinyl butyrals with low solution viscosity from DE-A 19641064, obtained via acetalization of a copolymer having vinyl alcohol units and 1-alkylvinyl alcohol units.
In the abovementioned three-stage process, polyvinyl acetals are prepared by preparing a solution of polyvinyl alcohol in water, treating this with acid, and precipitating the polyvinyl acetal from this solution (mother liquor) via addition of aldehydes. The product is separated from the mother liquor, where appropriate subjected to a further treatment, washed, and dried. To increase the degree of acetalization, the process of EP-B 513857 proposes carrying out the acetalization with use of a temperature gradient, i.e. increasing the temperature during the precipitation.